The rapid and effective repair of bone defects caused by injury, disease, wounds, or surgery is a goal of orthopedic surgery. Toward this end, a number of bone implants have been used or proposed for use in the repair of bone defects. The biological, physical, and mechanical properties of the bone implants are among the major factors influencing their suitability and performance in various orthopedic applications.
Bone implants are used to repair bone that has been damaged by disease, trauma, or surgery. Bone implants may be utilized when healing is impaired in the presence of certain drugs or in disease states such as diabetes, when a large amount of bone or disc material is removed during surgery, or when bone fusion is needed to create stability. In some types of spinal fusion, for example, bone implants are used to replace the cushioning disc material between the vertebrae or to repair a degenerative facet joint.
One type of bone implant is the bone graft. Typically, bone graft (e.g., osteograft) materials may include both synthetic and natural bone. Natural bone may be taken from the graft recipient (autograft) or may be taken from another source (allograft), such as a cadaver, or (xenograft), such as bovine. Autografts have advantages such as decreased immunogenicity and greater osteoinductive potential, but there can also be problems with donor site morbidity and limited supply of suitable bone for grafting. On the other hand, allografts are available in greater supply and can be stored for years. However, allografts tend to be less osteoinductive.
Osteoconduction and osteoinduction both contribute to bone formation. A graft material is osteoconductive if it provides a structural framework or microscopic and macroscopic scaffolding for cells and cellular materials that are involved in bone formation (e.g., osteoclasts, osteoblasts, vasculature, mesenchymal cells).
Osteoinductive material, on the other hand, stimulates differentiation of host mesenchymal cells into chondroblasts and osteoblasts. Natural bone allograft materials can comprise either cortical or cancellous bone. A distinguishing feature of cancellous bone is its high level of porosity relative to that of cortical bone, providing more free surfaces and more of the cellular constituents that are retained on these surfaces. It provides both an osteoinductive and osteoconductive graft material, but generally does not have significant load-bearing capacity. Optimal enhancement of bone formation is generally thought to require a minimum threshold quantity of cancellous bone, however. Cortical (compact) bone has greater strength or load-bearing capacity than cancellous bone, but is less osteoconductive. In humans for example, only approximately twenty percent of large cortical allografts are completely incorporated at five years. Delayed or incomplete incorporation may allow micromotion, leading to host bone resorption around the allograft. A more optimal bone graft material would combine significant load-bearing capacity with both osteoinductive and osteoconductive properties, and much effort has been directed toward developing such a graft material.
Some allografts comprise mammalian cadaver bone treated to remove all soft tissue, including marrow and blood, and then textured to form a multiplicity of holes of selected size, spacing, and depth. The textured bone section is then immersed and demineralized, for example, in a dilute acid bath. Demineralizing the bone exposes osteoinductive factors, but extensive demineralization of bone also decreases its mechanical strength.
Allografts have also been formed of organic bone matrix with perforations that extend from one surface, through the matrix, to the other surface to provide continuous channels between opposite surfaces. The organic bone matrix is produced by partial or complete demineralization of natural bone. Although the perforations increase the scaffolding potential of the graft material and may be filled with osteoinductive material as well, perforating organic bone matrix through the entire diameter of the graft decreases its load-bearing capacity.
Partially-demineralized cortical bone constructs may be surface-demineralized to prepare the graft to be soaked in bone growth-promoting substances such as bone morphogenetic protein (BMP). Although this design allows greater exposure of the surrounding tissue to growth-promoting factors, the surface demineralization necessary to adhere a substantial amount of growth-promoting factors to the graft material decreases the allograft's mechanical strength.
What is needed is a bone implant that combines the osteoinductive and osteoconductive properties of cancellous bone with the load-bearing capacity provided by cortical allograft materials. Compositions and methods are needed that facilitate bone remodeling and new bone growth, and integration of the bone implant (e.g., allograft) into host bone.